The Chargers rookie wide receiver knows he made a mistake. The fallout of it, he said Monday, caught him off guard.

Allen, a third-round draft pick, posted a Vine video last week on his Twitter account. It showed a recent image of him wearing a Raiders hat while sipping a drink at an In-N-Out restaurant.

The image went viral, attracting national headlines and several dozen online messages from Chargers fans less than thrilled to see the 21-year-old sporting the gear of a division rival.

"I didn't think it would have the effect that it did have," said Allen, who first apologized to fans Saturday on Twitter. "But obviously, it did. I didn't mean for it to happen. I wasn't thinking about it at the time. I wasn't actually thinking about football at all. I just wanted to grab a bite to eat. I posted something online, and I'm sorry about it. It won't happen again."

Allen was in Berkeley last week for California's final exams, so he missed the start of the Chargers' organized team activities.

The team worked him back slowly Monday, holding him out of most activities. His activity level is expected to increase Tuesday.

Allen said he's learned his lesson about the attire he chooses. Just in case, one of the Chargers' most respected veterans made sure.

"Eric (Weddle) came up to me today," Allen said. "He gave me the rundown: 'Don't do it again.' It won't happen again."